Resistive Temperature Detectors (RTDs) are sensors that provide an electrical resistance that is indicative of a temperature. RTDs have relatively-high signal-sensitivity compared with other types of temperature sensors. The electrical signal generated by RTDs are repeatable, in that if an RTD is repeatedly exposed to the same temperature, then the RTD will repeatedly generate substantially the same electrical signal in response. RTDs are made using materials that have a resistivity that is a function of temperature. For example, some Metallic RTDs, such as platinum, exhibit a positive temperature coefficient of resistance.
Although RTDs are repeatable and have relatively-high signal-sensitivity, RTDs can produce signals that are inaccurate. For example, RTDs have a non-zero thermal mass. Because the thermal mass of RTDs is non-zero, the temperature of the RTD will lag dynamic external temperatures exposed thereto. Therefore, the more rapidly the dynamic external temperature varies, the greater will be the inaccuracy of the RTD. Other inaccuracies can result from self-heating within an RTD. For example, an RTD is typically biased by a current or by a voltage. If biased by a current, then the voltage across the RTD can be used as the electrical signal indicative of the temperature. If, however, biased by a voltage, then the current through the RTD can be used as the signal indicative of the temperature. In either scenario, the RTD will dissipate power, and some self-heating of the RTD will result.
Steady-state errors can also arise because of heat transfer dynamics of a temperature sensing system. For example, competing paths of thermal conductivity in a body of the temperature sensing system can prevent the sensing element of the system from reaching thermal equilibrium with the medium to be measured.
Thermocouples can be used instead of RTDs as an alternative temperature sensor. Thermocouples can be made to have much smaller thermal mass than many RTDs. Because thermocouples have very small thermal masses, thermocouples do not lag dynamic temperature variations by very much. Thermocouples also dissipate very little power. Thermocouples are typically not biased with a current or a voltage. Thermocouples are typically made of two electrically conductive wire leads of dissimilar materials. The wire leads can be connected to one another at a thermocouple junction. A temperature difference between the thermocouple junction and the unconnected ends of the wire leads results in a voltage across the unconnected ends of the wire leads. A high impedance voltage sensor can detect the resulting voltage while causing very little current to flow through the thermocouple. The thermocouple output signal is related to the temperature difference between the two junctions.
Although thermocouples have low self-heating and fast response times, thermocouples typically are less accurate and are less repeatable than RTDs. Thus, different applications might require, or at least prefer, different temperature sensors.